In modern marine seismic streamer systems, a vessel tows a long cable with a large number of sensors. Recent developments in such systems have simultaneously focused on making them light, durable, easy to manufacture and maintain, as well as sensitive to the acoustic signals of interest while remaining relatively immune to noise. These developments have lead to improvements in sensor elements, such as those disclosed in U.S. Pat. Nos. 5,541,894; 5,663,931; 5,677,894; and others. The sensor elements disclosed in these patents have been increasingly durable and eliminate the long vexing problem of distortion due to harmonics.
Another improvement in sensor technology was disclosed in U.S. Pat. No. 5,632,841 to Hellbaum et al. which teaches a method for forming piezoelectric wafers including a pre-stress layer. The method of Hellbaum et al. results in an arcuate sensor which is particularly sensitive to an acoustic signal.
The focus of U.S. patent application Ser. No. 09/292,915 allowed, the parent application to the present application, was a method and structure for incorporating the arcuate sensor of Hellbaum et al. into a manufacturable marine seismic streamer. Such a structure may use the arcuate sensor element of Hellbaum et al., or it may use more conventional flat or domed diaphragm elements with a conventional flat piezoelectric sensor.
Testing of the structure in the parent application has proved the efficacy of the structure disclosed therein, and has resulted in certain improvements and refinements, which are the focus of the present application. More particularly, we have found that a presumably acoustically inert jacket over a hydrophone sensor element in fact enlarges the acoustic aperture of the sensor. This discovery has lead to the development of the embodiments disclosed herein.